The present invention is concerned with wing tip device attachment apparatus and method. More particularly, the present invention is concerned with an apparatus and method for attaching a wing tip device such as a winglet to the tip of a passenger aircraft wing.
Wing tip devices are well known in the art. Devices such as winglets, raked wing tips and fences are collectively known as aerodynamic wing tip devices and are used to reduce the effects of lift induced drag.
Lift induced drag is caused by the generation of vortices at the wing tip. Such drag is mitigated by an increase in wing span. Increases in wing span in the plane of the wing are not always possible due to space requirements at, e.g. airports. As such out-of-plane extensions to the wing are commonly used to increase the effective wing span without increasing the geometric span of the aircraft. These take the form of aerodynamic wing tip devices.
There is an ever increasing drive to increase the efficiency of passenger aircraft. One way to achieve increased efficiency is to increase the size of the aerodynamic wing tip device. Typical ratios of winglet span to the thickness of the wing tip at the attachment point (wingbox thickness) are commonly above 10 and may be as high as 12 to 15 in modern passenger aircraft. Because the thickness of the wing is low at the tip, the vertical moment arm available to react to the loads generated by the wing tip device under both its own weight and under aerodynamic forces is low. Therefore, the forces and stresses generated in this area are high.
Other wing tip devices include external tanks, refueling pods etc whose primary purpose is not to improve the aerodynamic efficiency of the wing, but nerveless are attached to, and produce a force on, the wing tip.
Known wing tip devices are generally attached in one of two manners. The first is to use a series of splice plates or butt straps which span the upper and lower skins of the wing tip device and the wingbox at the point at which they join.
The second method is to use abutting plates joined by tension bolts.
Disadvantageously, both of these methods only utilise a very small moment arm to react the loads. The splice plates transfer load through the wing skin, which is primarily designed to absorb the bending load across the wing span, and less well suited to absorb local, concentrated, loads. As such, the large local load introduced into the wing skin requires structural reinforcement. The additional weight this causes is undesirable.
Further, because the wing skin is not particularly strong, many joining locations are required to spread the applied load. Although this has the desired effect of reducing the load per joining location, it creates a statically indeterminate system making the loads at each point difficult to predict. Therefore each joint is typically over-engineered adding weight and cost to the aircraft.
Cyclic loading is common in aircraft. This introduces additional structural requirements, in particular to the tension bolt design. In order to mitigate the effects of fatigue, the bolts have to be pre-tensioned with an interference fit. This is undesirable as it adds complexity to the manufacturing process, and makes maintenance and replacement more difficult
A still further problem with the above two methods is that because of the heavy bolting and large surface area of contact between the various components in both methods, the interface between the parts is quite sensitive to differences in geometry at the interface. As such, any mis-match between the two components needs to be addressed with fettling upon assembly. This increases the cost of assembly and makes it more difficult to replace the wing tip devices in service.
Further, temperature effects and loading in use can cause differential expansion/contraction of the wing tip device and the wing tip, which can cause high stresses at the mounting points.